Method of sintering electrically conducting powders and an apparatus for carrying out said method

ABSTRACT

A method of sintering electrically conducting powders in an air atmosphere for obtaining a sintered product includes the following step sequence: placing the powders in an electrically isolating mold, applying a pressure to the powders between 100 and 500 MPa, and applying to the powders a sintering current at a sintering voltage during a sintering time, for sintering the powders. Before applying the sintering current density to the powders, an activation current density is lower than the sintering current density at an activation voltage greater than the sintering voltage during an activation time lower than the sintering time, to reduce the electrical resistance of the powders.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to and claims the benefit of Spanish Patent Application No. EP16382069, filed on Feb. 19, 2016, the contents of which are herein incorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to a method for obtaining sintered electrically conducting parts, which includes the application of heat and pressure to powders for finally obtaining dense parts, wherein the heat is obtained via electrical currents that are forced to pass through conductive powders.

BACKGROUND

ECAS (electric current assisted sintering) gathers a family of consolidation methods in which mechanical pressure is combined with electric and thermal fields to enhance interparticle bonding and densification. The starting materials can be in the form of either powders or green compacts. The primary purpose of imposed electric currents is to provide the required amount of resistive heat.

ECAS techniques can be classified with respect to the discharge time. Conventionally, 0.1 s discharge time can be assumed as the threshold between fast and ultrafast ECAS. However, confusion should be avoided between fast termed herein and the FAST acronym (field activated/assisted sintering technique) frequently encountered in the scientific literature. Here, fast simply refers to either a high processing rate or a low processing time.

On one hand, fast ECAS techniques are known with different acronyms mainly depending on the electric current waveform: SPS (Spark Plasma Sintering), PECS (Pulsed Electric Current Sintering), RS (Resistance Sintering), etc. The bases of the process were developed by INOUE (U.S. Pat. No. 3,241,956 A). The electric current is normally applied as pulses and also was reported the possibility of the application of various superimposed currents with different frequencies and two stages of pressure to increase densification. The application of the load was also described in U.S. Pat. No. 3,508,029 A. These two processes are characterized by the low intensity of the electric pulses (<1 kA/cm²) and high duration of the cycles (from seconds to minutes). In addition, the manufacturing of fast ECAS equipment is complex with the need of protective atmosphere (or vacuum).

On the other hand, ultrafast ECAS techniques typically employ either one or up to three repeated (capacitor) discharges. Each discharge lasts less than 0.1 s. The current pulse density can be on the order of 10 kNcm². Ultrafast ECAS is generally referred to as electric discharge compaction (EDC) or EFS (Electro Forging Sintering). Representative examples of these methods were explained in the following patents: EP 2198993 A1 from Fais, U.S. Pat. No. 4,929,415 and U.S. Pat. No. 4,975,412 from Okazaki. Main problem of these methods is that the capacitors discharge the stored energy in a sudden an uncontrolled way and thus they did not permit tailoring of the power input to the powder mass. The use of high current and high voltage resulted in inconsistent densification and inhomogeneity of parts manufactured using these consolidation processes, because the resistance of the powders is not homogeneous due to porosity, surface oxidation, compaction or bonding between the particles, and it is well known that the current always follows the lowest resistive path. Other problems are the low size of the samples that can be produced and the sparks in the electrodes produced by the high current and high voltage.

Other ultrafast processes used low voltage equipment like the patents developed by Cremer (U.S. Pat. No. 2,355,954), Knoess (U.S. Pat. No. 5,529,746) and Bauer (U.S. Pat. No. 7,361,301). Knoess and Bauer obtained good densification with high conductive powders like iron and copper. Knoess used various pulses of very high current (>100 kA/cm²) and Bauer an intensity lower that 10 kNcm², voltage lower than 10V using a sintering time around 1 s. The problem may occur when high electrical resistance samples are manufactured (due to the high resistivity of the powder or because of the large size of the parts), it will not be possible to close the electric circuit so the current can pass the whole material of the parts. For that reason, with these techniques it will not be possible to obtain larger samples or the consolidation of powders with higher resistivity due to the low voltage used.

SUMMARY

To overcome the drawbacks of the prior art, the present disclosure proposes a method of sintering electrically conducting powders in an air atmosphere for obtaining a sintered product, comprising the following sequence of steps:

-   -   a) placing the powders in an electrically isolating mold,     -   b) applying a pressure to the powders between 100 and 500 MPa,         and     -   c) applying to the powders a sintering current density at a         sintering voltage during a sintering time, for sintering the         powders.

The method of the present disclosure comprises, between step b) and step c), applying to the powders an activation current density lower than the sintering current density at an activation voltage greater than the sintering voltage during an activation time lower than the sintering time, to reduce the electrical resistance of the powders.

The application of the activation current density and sintering current density is carried out while the pressure is being applied to the powders.

The activation current density applied to the powders at a voltage greater than the voltage used for sintering in step c) during a lower time than step c) produces a current discharge that breaks the oxide layer in the surface of the powders and creates bridges between the particles of powders, obtaining a more uniform and cleaner particle surface which reduces the electrical resistance to the flow of the current through the powders such that the sintering current density applied in step c) is distributed more homogeneously through the powders in the mold. Thus, it is possible to sinter large size parts and parts made of material with a high electrical resistivity.

Preferably the activation current density is greater than 0,5 kA/cm² , the activation voltage is greater than 10V and the activation time is lower than 300 ms, for generating a current discharge of low intensity at a high voltage in a very reduced time, to assure an homogeneous superficial de-oxidation of the powders and formation of bridges among particles.

According to the disclosure, the time between the removal of the activation current density and the application of the sintering current density is lower than 20 ms to assure an optimal distribution of the sintering current density. Most preferably, the sintering current density is applied immediately after the application of the activation current density, i.e just after the activation time is run out.

According to a preferred aspect of the disclosure, the activation current density and the sintering current density are constant.

According to a preferred aspect of the disclosure:

-   -   The activation current density is applied by employing a first         electrical power unit.     -   The sintering current density is applied by employing a second         electrical power unit.     -   The first and second electrical power units are operated         independently.

The method of the disclosure comprises the control of the two electrical power units which enables to optimize the processing time and the energy consumption, altogether with the installation costs.

Further it allows to program and monitor at all times the power that is being introduced in the powder, thus allowing the process to be controlled very accurately and repetitively, both in the application of the activation current density and in the application of the sintering current density.

Furthermore, it has been found that the precise control enabled by this method, allows a considerable increase in the parts size, their geometry, and the types of materials that can be sintered.

According to a preferred embodiment of the disclosure:

-   -   The activation current density is comprised in the range going         from 0.5 to 5 kA/cm²;     -   The activation voltage is comprised in the range going from 10         to 100 V;     -   The activation time is comprised in the range going from 50 to         300 ms;     -   The sintering current density is comprised in the range going         from 3 to 15 kA/cm²;     -   The sintering voltage is lower than 15 V;     -   The sintering time is comprised in the range going from 500 to         1500 ms,

but these parameters must fulfil the conditions that the selected values for the sintering current density applied to the powders must be greater than the selected values for the activation current density and the selected values for the activation voltage always must be greater than the sintering voltage applied.

According to a most preferred embodiment of the disclosure:

-   -   The activation current density is comprised in the range going         from 0.5 to 4 kA/cm²;     -   The activation voltage is comprised in the range going from 10         to 100 V;     -   The activation time is comprised in the range going from 90 to         200 ms;     -   The sintering current density is comprised in the range going         from 3 to 10 kA/cm²;     -   The sintering voltage is lower than 10 V;     -   The sintering time is comprised in the range going from 500 to         1500 ms,

but these parameters must fulfil the conditions that the selected values for the sintering current density applied to the powders must be greater than the selected values for the activation current density and the selected values for the activation voltage always must be greater than the sintering voltage applied.

The skilled person will select the precise values of these parameters for each conductive powder, but always bearing in mind that the activation current density applied to one conductive powder must be lower than the sintering current density applied and the activation voltage greater than the sintering voltage. Thus it is not possible to choose a value of 4 kA/cm² for the activation current density and a value of 3 kNcm²for the sintering current density. Same applies to the activation and sintering voltages where is not possible to apply a activation voltage of 10 V and a sintering voltage of 15 V. Some examples of parameters selections are disclosed in the description of preferred embodiments.

The disclosure also relates to an apparatus for carrying out the sintering of electrically conducting powders in an air atmosphere, comprising an electrically isolating mold which can be filled with the powders, two opposite electrodes coupleable to the mold, means for applying a pressure to the powders in the mold and means for providing current and voltage through the electrodes. According to the disclosure the means for providing current and voltage comprise:

-   -   a first electrical power unit arranged to provide to the         powders, through the electrodes, an activation current density         at an activation voltage (activation step);     -   a second electrical power unit arranged to provide to the         powders, through the electrodes, a sintering current density at         a sintering voltage (sintering step);

the activation current density being lower than the sintering current density and the activation voltage being greater than the sintering voltage.

According to the disclosure the apparatus further comprises:

-   -   Means for switching between the first and the second electrical         power unit;     -   Means for controlling the duration (activation time) of the         activation current density and activation voltage provided by         the first power unit;     -   means for controlling the duration (sintering time) of the         sintering current density and sintering voltage provided by the         second power unit;

the sintering time being greater than the activation time.

This device, specially designed for carrying out the method of the disclosure can be implemented with low cost commercial equipment.

In an embodiment of the disclosure the first power unit is configured for providing an activation current density greater than 0.5 kA/cm² and an activation voltage greater than 10V during an activation time lower than 300 ms, for generating a current discharge of low intensity at a high voltage in a very reduced time, to assure an homogeneous superficial de-oxidation of the powders.

In a preferred embodiment of the disclosure the first power unit is configured for providing an activation current density between 0.5 and 5 kA/cm² and an activation voltage between 10 and 100 V and the second power unit is configured for providing a sintering current density between 3 and 15 kA/cm² and an activation voltage lower than 15 V.

According to a preferred embodiment the apparatus of the disclosure comprises switching means which allows to control the activation and deactivation of the first and second electrical power unit so that the sintering current density is applied preferably immediately after the removal of the activation current density, i.e just after the activation time is run out, and preferably the time between the deactivation of the first electrical power unit and the activation of the second electrical power unit is lower than 20 ms to assure an optimal distribution of the sintering current density.

The first power unit can be configured for providing a constant activation current density or a constant activation voltage, and the second power unit can be configured for providing a constant sintering current density.

The means for controlling the duration of the activation current density and activation voltage provided by the first power unit can control a predetermined discharge time (activation time) comprised in the range going from 50 to 300 ms;

The means for controlling the duration of the sintering current density and sintering voltage provided by the second power unit can control a predetermined discharge time (sintering time) comprised in the range going from 500 to 1500 ms;

In a preferred embodiment each power unit comprises a transformer and an inverter.

Preferably the two power units are controlled by a control unit and the control unit is a programmable logic controller.

The control unit can comprise:

-   -   means for switching between the first and the second electrical         power unit;     -   means for controlling the duration of the activation current         density and activation voltage provided by the first power unit;     -   means for controlling the duration of the sintering current         density and sintering voltage provided by the second power unit.

BRIEF DESCRIPTION OF THE DRAWINGS

To complete the description and in order to provide for a better understanding of the disclosure, a set of drawings is provided. Said drawings form an integral part of the description and illustrate an embodiment of the disclosure, which should not be interpreted as restricting the scope of the disclosure, but just as an example of how the disclosure can be carried out. The drawings comprise the following figures:

FIG. 1 is a block diagram of an apparatus according to a preferred embodiment; and

FIG. 2 is a time plot of the pressure and the voltage/current when the inventive method is applied to a WC—Co powder.

DETAILED DESCRIPTION OF THE DRAWINGS

In FIG. 2 the time pressure/current/voltage diagram corresponding to the implementation of the method according to the disclosure for obtaining a sintered WC—Co is shown.

The process starts with the step comprising placing an electrically conducting powder in an electrically insulating mold.

Then a pressure between 100 and 500 MPa is applied inside the mold, preferably with two pistons, in this case around 300 MPa.

Then an activation step is carried out, comprising applying an activation current density at an activation voltage for an activation time and carried out by employing a first electrical power unit (2). As shown, in this step a low current density (around 2 kA/cm2) and a high voltage (around 30V) are applied. The pulse is about two tenths of a second.

Then a waiting step is carried out wherein no current and/or voltage are applied. This step comprises switching of the power units, that is, to switch from a power unit (2) to another power unit (3). The waiting time is that needed for carrying out said switching by the control unit (4), in the present case a PLC. In FIG. 2 the switching time is about 2 tenth of a second. A technical possibility would be to use a single power unit but with instantaneously variable current and voltage. However, the control requirements for the current and voltage levels and the discharge times would imply very sophisticated equipment that would make the method uneconomic at industrial level.

Then, the proper sintering step is performed, which comprises applying a sintering current density at a sintering voltage during a sintering time carried out by employing the second electrical power unit (3). In this case the intensity is higher (around 10 kA), but the voltage is reduced to 5 V.

The current density is applied using two opposite electrodes. In an embodiment the pistons can be used as opposite electrodes.

As shown in FIG. 1, and according to a preferred embodiment, the disclosure also relates to an apparatus (1) for carrying out the inventive method.

The apparatus comprises:

-   -   means for applying current and voltage to the powders,         represented by the power units (2, 3);     -   an electrically insulating mold (5) containing the conductive         powders (6), which is closed in its ends by two pistons for         applying mechanical pressure and which form the electrodes (7)         as well.

As shown in FIG. 1, the means for providing current and voltage for an activation step is a first electrical power unit (2) and the means for providing current and voltage for a sintering step is a second electrical power unit (3).

The first power unit (2) is arranged to provide through the electrodes (7) an activation current density comprised between 0.5 and 5 KA/cm² and an activation voltage comprised between 10 and 100 V whereas the second power unit (3) is arranged to provide through electrodes (7) a sintering current density comprised between 3 and 15 kA/cm² and a sintering voltage lower than 15 V. These ranges allow to sinter most of the commercially interesting conductive powders for typical applications, with a single machine, which parameters have to be set prior to the sintering.

The apparatus further comprises:

-   -   Means for switching between the first 2 and the second 3         electrical power unit;     -   Means for controlling the duration of the current density and         voltage provided by the first power unit (2);     -   means for controlling the duration of the current density and         voltage provided by the second power unit (3);     -   connections (23, 33) between each of the power units (2, 3) and         the electrodes (7) of the mold (5).     -   means for controlling the pistons that apply pressure in the         mold.

The means for controlling the duration of the current density and voltage provided by the first power unit (2) are able to control a predetermined discharge time (activation time) comprised in the range going from 50 to 300 ms and the means for controlling the duration of the current and voltage provided by the second power unit (3) are able to control a predetermined discharge time (sintering time) comprised in the range going from 500 to 1500 ms.

Each power unit (2, 3) comprise a transformer (21, 31) and an inverter (22, 32), and the two power units (2, 3) are controlled by a single control unit (4), which is preferably a programmable logic controller.

This PLC includes:

-   -   means for switching between the first (2) and the second (3)         electrical power unit,     -   means for controlling the duration of the current and voltage         provided by the first power unit (2),     -   means for controlling the duration of the current and voltage         provided by the second power unit (3); and     -   means for controlling the pistons that apply pressure in the         mold.

Now, specific examples of application of the method of the disclosure to different metal powders are described.

EXAMPLE 1 WC-6Co/WC-10Co

A WC-6Co or WC-10Co disk is produced with the disclosed apparatus with a thickness of 16 mm and a diameter of 22 mm. The agglomerated powder was spherical with an agglomerate size of less than 100 microns.

In the activation step a current density between 2 and 4 kA/cm² during 100-200 ms was applied in order to activate the powder. A voltage between 15-50 V is needed for this activation step.

In the subsequent sintering stage a current density between 6-10 kNcm² was applied to obtain a densified sample with a voltage lower than 10 V during 500-1000 ms. Between stages, activation and sintering, a minimum time of 10 ms was established. Pressure, from 100-500 MPa, was applied from the beginning of the process.

The density of the final disk, measured by the Archimedes method, is around 13-14.8 g/cm³. It is possible to obtain fully dense samples with hardness around 1800-2100 HV30.

EXAMPLE 2 Titanium

A titanium disk is produced with the disclosed apparatus with a thickness of 10 mm and a diameter of 22 mm. The shape of the particles of the powder was irregular with a maximum particle size around 75 microns.

In the activation step a current density between 1-3 kNcm² was applied during 90-100 ms in order to activate the powder. A voltage between 10-50 V is needed for the activation stage.

In the sintering stage a current density between 4-7 kA/cm² was applied during 500-1000 ms to obtain a densified sample with a voltage lower than 10 V. Between stages, activation and sintering, a minimum time of 10 ms was established. Pressure, from 100-500 MPa, was applied from the beginning of the process.

The density of the final disk, measured by the Archimedes method, is around 3.5-4.4 g/cm³. It is possible to obtain fully dense samples.

EXAMPLE 3 TiC-25Ni and TiC-25Fe

A TiC-25Ni and TiC-25Fe disks are produced with the disclosed apparatus with a thickness of 16 mm and a diameter of 22 mm. The agglomerated powder was irregular with a particle size of less than 30 microns.

In the activation step a current density between 1-3 kA/cm² was applied during 100-200 ms in order to activate the powder. A voltage between 15-50 V is needed for this activation stage.

In the subsequent sintering step a current density between 6-9 kA/cm² was applied during 500-1000 ms to obtain a densified sample with a voltage lower than 10 V. Between stages, activation and sintering, a minimum time of 10 ms was established. Pressure, from 100-500 MPa, was applied from the beginning of the process.

The density of the final disk, measured by the Archimedes method, was around 5.1-5.5 g/cm³ for TiC-25Ni and 5.1-5.4 g/cm³ for TiC-25Fe. It is possible to obtain fully dense samples with hardness around 1600-2000 HV30.

EXAMPLE 4 Aluminium

An aluminium disk was produced with the disclosed apparatus with a thickness of 12 mm and a diameter of 12 mm. The powder was irregular with a particle size of less than 150 microns.

In the activation step a current density between 0.5-2 kNcm² was applied during 100-200 ms in order to activate the powder. A voltage between 30-80 V is needed for this activation stage.

In the subsequent sintering stage a current density between 3-4 kNcm² was applied during 500-1000 ms to obtain a densified sample with a voltage lower than 10 V. Between stages, activation and sintering, a minimum time of 10 ms was established. Pressure, from 100-300 MPa, was applied from the beginning of the process.

The density of the final disk, measured by the Archimedes method, was around 2.5-2.7 g/cm³.

The disclosure is obviously not limited to the specific embodiments described herein, but also encompasses any variations that may be considered by any person skilled in the art. 

1. A method of sintering electrically conducting powders in an air atmosphere for obtaining a sintered product, the method including the following sequence of steps: a) placing the powders in an electrically isolating mold, b) applying a pressure to the powders between 100 and 500 MPa, and c) applying to the powders a sintering current at a sintering voltage during a sintering time, for sintering the powders, wherein applying to the powders, between step b) and step c), an activation current density lower than the sintering current density at an activation voltage greater than the sintering voltage during an activation time lower than the sintering time, to reduce the electrical resistance of the powders.
 2. The method according to claim 1, wherein the activation current density is greater than 0.5 kA/cm², the activation voltage is greater than 10V, and the activation time is lower than 300 ms.
 3. The method according to claim 1, wherein: the application of the sintering current density and sintering voltage are carried out by employing a first electrical power unit; the application of the activation current density and activation voltage are carried out by employing a second electrical power unit; and the first and second electrical power units operate independently.
 4. The method according to claim 1, wherein: the activation current density is comprised in the range of 0.5 to 5 kA/cm²; the activation voltage is comprised in the range of 10 to 100 V; the activation time is comprised in the range of 50 to 300 ms; the sintering current density is comprised in the range of 3 to 15 kA/cm²; the sintering voltage is lower than 15 V; the sintering time is comprised in the range of 500 to 1500 ms, and wherein the activation current density is lower than the sintering current density and the activation voltage is greater than the sintering voltage.
 5. The method according to claims 1, wherein: the powders are WC-6Co powders or WC-10Co; the activation current density is comprised in the range of 2 to 4 kA/cm²; the activation voltage is comprised in the range going from of 15 to 50 V; the activation time is comprised in the range going from of 100 to 200 ms; the sintering current density is comprised in the range of 6 to 10 kA/cm²; the sintering voltage is lower than 10 V; and the sintering time is comprised in the range of 500 to 1000 ms.
 6. The method according to claim 1, wherein: the powders are titanium powders; the activation current density is comprised in the range of 1 to 3 KA/cm²; the activation voltage is comprised in the range of 10 to 50 V; the activation time is comprised in the range of 90 to 110 ms; the sintering current density is comprised in the range of 4 to 7 kA/cm²; the sintering voltage is lower than 10 V; and the sintering time is comprised in the range of 500 to 1000 ms.
 7. The method according to claim 1, wherein: the powders are a mixture of TiC-25Ni powders and TiC-25Fe powders; the activation current density is comprised in the range of 1 to 3 kA/cm²; the activation voltage is comprised in the range of 15 to 50 V; the activation time is comprised in the range of 100 to 200 ms; the sintering current density is comprised in the range of 6 to 9 kA/cm²; the sintering voltage is lower than 10 V; and the sintering time is comprised in the range of 500 to 1000 ms.
 8. The method according to claim 1, wherein: the powders are an aluminium powders; the activation current density is comprised in the range of 0.5 to 2 kA/cm²; the activation voltage is comprised in the range of 30 to 80 V; the activation time is comprised in the range of 100 to 200 ms; the sintering current density is comprised in the range of 3 to 4 kA/cm²; the sintering voltage is lower than 10 V; and the sintering time is comprised in the range of 500 to 1000 ms.
 9. An apparatus for carrying out the sintering of electrically conducting powders in an air atmosphere, comprising an electrically isolating mold which can be filled with the powders, two opposite electrodes coupleable to the mold for applying a current density to the powders in the mold, means for applying a pressure to the powders in the mold and means for providing current and voltage through the electrodes wherein the means for providing current density and voltage comprise: a first electrical power unit arranged to provide to the powders a activation current density at an activation voltage; a second electrical power unit arranged to provide to the powders a sintering current density at a sintering voltage; the activation current density being lower than the sintering current density and the activation voltage greater than the sintering voltage; the apparatus further comprising: means for switching between the first and the second electrical power unit; means for controlling the duration of the activation current density and activation voltage provided by the first power unit; means for controlling the duration of the sintering current density and sintering voltage provided by the second power unit; and the activation time being lower than the sintering time.
 10. The apparatus according to claim 9, wherein: means for controlling the duration of the activation current density and activation voltage provided by the first power unit are able to control an activation time comprised in the range of 50 to 300 ms; and means for controlling the duration of the sintering current density and sintering voltage provided by the second power unit are able to control a sintering time comprised in the range of 500 to 1500 ms.
 11. The apparatus according to claim 9, wherein the first power unit is able to provide a activation current density greater than 0.5 kA/cm² and an activation voltage greater than 10 V.
 12. The apparatus according to claim 9, wherein the first power unit is able to provide an activation current density between 0.5 and 5 kA/cm² and an activation voltage between 10 and 100 V, and the second power unit is able to provide a sintering current density between 3 and 15 kA/cm² and a sintering voltage lower than 15 V.
 13. The apparatus according to claim 9, wherein each power unit comprises a transformer and an inverter.
 14. The apparatus according to claim 9, wherein the two power units and the means for applying a pressure to the mold are controlled by a control unit, preferably a programmable logic controller.
 15. The apparatus according to claim 14, wherein the control unit includes: means for switching between the first and the second electrical power unit; means for controlling the duration of the activation current density and activation voltage provided by the first power unit; and means for controlling the duration of the sintering current density and sintering voltage provided by the second power unit. 